The present invention relates to lampholders for receiving bulbs which are screwed in, such bulbs being generally known as Edison bulbs.
Conventional Edison type lampholders comprise a body supporting fixed terminals to which electrical cables are connected, the body defining a threaded socket into which a lamp bulb is in use screwed. The threaded portion of the socket constitutes one contact of the lampholder, the other contact being positioned axially within the socket so that the tip of the bulb screwed into the socket presses against it. Generally the contacts are permanently electrically connected to the fixed terminals.
When no bulb is present in the socket the contacts are exposed and can be easily touched. This can happen for example when replacing a faulty bulb in an overhead lampholder, or by accidentally putting ones finger in an empty socket from which a faulty bulb has been removed. Children are particularly at risk in the latter case when bedside or other free standing lampholders are provided.
Conventional three pin socket outlets into which cable terminating plugs are inserted generally have their live and neutral terminals covered by a shutter, the shutter being pulled away when an earth pin of a plug is inserted into the earth terminal socket. Such socket outlets are relatively safe as if one is to electrically contact the live terminal a pin must first be inserted into the earth terminal socket and then a further pin must be inserted into the live terminal socket. In contrast, in conventional lampholders not only are the live and neutral terminals not protected by a shutter but also the common socket in which they are located is large enough to enable a finger to be inserted.
The danger represented by conventional lampholders has been previously recognised and various attempts have been made to isolate the contacts within the socket from the cable terminals except when a bulb is present in the socket.
In one known lampholder described in European Patent Specification No. 0 077 046 a movable carrier is provided which is engaged by the thread of an inserted bulb so as to move relative to the lampholder body when a bulb is screwed in. Movement of the carrier in response to insertion of the bulb causes contacts accessible in the lampholder socket to be connected to cable terminals otherwise isolated within the lampholder body. The contacts are moved apart axially with respect to the lampholder socket so that arcing can occur if a bulb is inserted when the lampholder is connected to an electrical supply. Such arcing can seriously damage the lampholder.
It is an object of the present invention to provide an alternative safety lampholder for Edison type bulbs to that described in European Patent Specification No. 0 077 046.